A grocery store cashier has to say "Hello", (maybe) answer a question about a price or something on special, scan the items, get the credit card signed or take cash and make change, put the items in a bag, say “Thank you”, and hand the bag(s) to the customer. That’s their JOB.

Yes, but the cashier earns at least minimum wage, not the several-dollars-below-minimum that servers earn. The cashier may have a line, but is not waiting on several other groups while helping the customer. And the cashier is not assessed a certain amount of tips as taxable income by the IRS based on the amount of sales he or she makes, regardless of whether or not she or he actually gets tipped on that amount.